How big is the Moon?

The Moon's equatorial diameter is 3,476 km and its mean radius is 1,737.4 km. By comparison, that is less than a third of Earth's width; NASA illustrates the difference by saying that if our planet were the size of a small coin, the Moon would be like a coffee bean. That scale, modest next to Earth, is unusually large for a satellite: most moons in the solar system are tiny relative to the world they orbit.

The average separation between Earth and the Moon is 384,400 km, but the orbit is elliptical and the figure ranges between 356,000 and 407,000 km. That variation explains why the lunar disk looks somewhat larger during some full moons than others, the effect the press has popularized as a "supermoon," and also why the apparent size changes just enough for some solar eclipses to be total and others annular.

With that mass and radius, surface gravity is 1.62 m/s², about a sixth of Earth's: hence the bounding walk of the Apollo astronauts and the fact that a rock sample weighs there a fraction of what it would on Earth. Escape velocity is 2.38 km/s, far lower than Earth's, which makes launching from the Moon dramatically cheaper and is one of the arguments for using it as a stepping stone to other destinations. The Moon also takes 27.3 days to orbit Earth once and another 27.3 days to rotate once on its own axis, a coupling that keeps the same face always turned toward us.